create a test program with libmodbus

modbus-rtu-master.c

/*
*---------------------------------------
*
*        modbus-rtu-master.c
*
*---------------------------------------
*/

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <modbus/modbus.h>

#define PORT_MODBUS     ("/dev/ttyUSB0")
#define PORT_BAUDRATE   (9600)
#define PORT_PARITY     ('N')
#define PORT_DATABIT    (8)
#define PORT_STOPBIT    (1)
#define SLAVE_ID        (16)

int main(int argc, char *argv[])
{
    uint16_t  tab_reg[64] = {0};
    modbus_t *ctx = NULL;

    int rc;
    int i;

    ctx = modbus_new_rtu(PORT_MODBUS,
                        PORT_BAUDRATE, PORT_PARITY, PORT_DATABIT, PORT_STOPBIT);

    if (ctx == NULL)
    {
        fprintf(stderr, "Unable to allocate libmodbus contex\n");
        return -1;
    }

    modbus_set_debug(ctx, 1);   // setting for debug message
    modbus_set_slave(ctx, SLAVE_ID);

    if (modbus_connect(ctx) == -1)
    {
        fprintf(stderr, "Connection failed:%s\n", modbus_strerror(errno));
            return -1;
    }

    while (1)
    {
        printf("\n----------------\n");
        #if 1
        // function 03
        rc = modbus_read_registers(ctx, 5, 7, tab_reg);
        if (rc == -1)
        {
            fprintf(stderr,"%s\n", modbus_strerror(errno));
            return -1;
        }
        else {
            for (i=0; i < rc; i++)
            {
                    printf("reg[%d] = %d(0x%x)\n", i, tab_reg[i], tab_reg[i]);
            }
        }
        #endif
        #if 0
        // function 04
        rc = modbus_read_input_registers(ctx, 0, 3, tab_reg);
        if (rc == -1)
        {
            fprintf(stderr,"%s\n", modbus_strerror(errno));
            return -1;
        }
        else {
            for (i=0; i < rc; i++)
            {
                    printf("reg[%d] = %d(0x%x)\n", i, tab_reg[i], tab_reg[i]);
            }
        }
        #endif
        sleep(1);
    }

    modbus_close(ctx);
    modbus_free(ctx);
    return 0;
}